As a vehicle moves forward, external air flow along surfaces of the vehicle separates from the surfaces at the aft end of the vehicle. Separation is most pronounced in vehicles with a substantially vertical aft face (e.g. trucks). The separation of flow from surface creates an area of low pressure behind the aft face of the vehicle. The area of low pressure “drags” the vehicle backward in a phenomenon known as aerodynamic drag. Aerodynamic drag on vehicles (e.g. trucks) significantly reduces fuel efficiency.
Reduced fuel efficiency leads to increased fuel consumption which contributes to air pollution and/or production of greenhouse gases and/or increased transportation cost.
Mechanical (static) flow deflectors mounted on vehicles in various configurations are employed to control external flow layer flow in an attempt to increase fuel efficiency. These mechanical flow deflectors are commonly referred to as “flarings” or “deflectors”.
Aerodynamic drag accounts for approximately 50%-70% of total motion resistance on a moving ground vehicle (e.g. truck) at highway speeds. Therefore, a 20% reduction in aerodynamic drag has the potential to a net reduction of total drag by 10%. The 10% drag reduction has the potential to reduce fuel consumption by 7-10%. Actual reductions in fuel consumption from a 20% reduction in aerodynamic drag are influenced by other factors, including, but not limited to, vehicle weight and energetic cost of implementing the reduction. At current fuel prices, a truck traveling 50,000 miles/year at highway speeds could realize a net savings of $3000 per annum from a 20% reduction in aerodynamic drag. Trucks traveling greater distances can realize a proportionately greater savings. Alternatively, or additionally, the 7-10% reduction in fuel consumption could have significant environmental impact. Existing passive deflectors do not have the capacity to reduce aerodynamic drag and/or fuel consumption to this degree. Additional development of passive deflectors to increase a degree to which they contribute to fuel efficiency is considered impractical because of limitations imposed by size and/or weight and/or cost and/or operational considerations. Operational considerations include, but are not limited to, effects of loading and unloading goods, obstructing visibility of lights and reflectors, and significant overhang of devices.
Means of producing super-circulation via application of high speed wall-jets to curved aerodynamic shapes using the Coanda effect have been extensively studied. (See for example: Jones, G S and Englar, R J, AIAA paper 2003-3411, the contents of which are fully incorporated herein by reference.) More recently similar concepts have been applied to reduce the drag of heavy vehicles. (See for example: SAE Technical paper 2001-01-2072 by R. J. Englar and AIAA paper 2004-2249 by R. C. McCallen et al., the contents of which are fully incorporated herein by reference.)
U.S. Pat. No. 4,736,913 to Bennett et al. describes a fluid flow control device which controllably maintains attached flow in the region of a body having a contour of rapid curvature utilizing tangential fluid discharge slots, positioned just upstream from the separation line, which issue a thin jet sheet to energize the external flow layer and entrain the surrounding flow. Bennett describes application to the aft fuselage of an aircraft to reduce separation and vortex drag at cruise and provide control forces and moments during low speed operation of the aircraft. The disclosure of this patent is fully incorporated herein by reference.
U.S. Pat. No. 7,104,498 to Englar et al. describes an aircraft comprising a channel wing having blown channel circulation control wings (CCW) for various functions. The described CCW relies upon a source of pressurized air and is capable of tangentially discharging pressurized air over the rounded trailing edge. The pressurized air being discharged over the rounded trailing edge provides a high lift that is obtained independent of an aircraft angle of attack, thus preventing the asymmetry, separated flow, and stall experienced by the CC wing at the high angle of attack it required for high lift generation. The aircraft can further include blown outboard circulation control wings (CCW) that are synergistically connected to the blown channel CCWs. The blown outboard CCWs provide additional high lift, control thrust/drag interchange, and can provide all three aerodynamic moments when differential blowing is applied front-to-rear or left-to-right. Both the blown channel CCW. The disclosure of this patent is fully incorporated herein by reference.
U.S. Pat. No. 7,055,541 to Seifert et al. entitled “Method and Mechanism for Producing Suction and Periodic Excitation Flow”, the disclosure of which is fully incorporated herein by reference, describes exemplary valves suitable for use in some embodiments of the invention disclosed herein.
In earlier studies to characterize the effect of external flow control on wing sections, it has been determined that oscillating or pulsed streams of air applied to or near the surface for the purpose of controlling separation can be as much as one hundred times more effective than similar flows applied without oscillation or pulsing. (Seifert et al. (1996) J. of Aircraft 33 (4): 691-699).